The present invention relates to waveguides and, in particular, it concerns a waveguide structure of polymer material on a non-polymer substrate and a corresponding production method.
Optical waveguides are structures that constrain or guide the propagation of light along a path defined by the physical construction of the waveguide. The dimensions of the waveguide in the direction in which the light is confined are on the order of the wavelength of the light. Such optical waveguides comprise a region of high refractive index in which most of the optical field of the light is located surrounded by regions of lower refractive index. Typically, an optical waveguide comprises a three-layer or sandwich structure comprising a substrate, a middle layer, and a top layer or cover. The top layer is frequently air. The index of refraction is largest in the middle layer within which the light is guided.
Optical waveguides may be constructed as a pattern of strips or channels on a substrate. The width and depth of the channels are on the order of the wavelength of the light to be guided, thus confining the propagation of light along the channels.
Channel waveguides may be made by a variety of different techniques. One of the simplest and most effective methods for making channel waveguides in glass is a technique known as ion exchange. According to this technique, a base glass containing, for example, sodium ions is covered with a metal mask. The glass may typically be an alkali aluminoborosilicate or a soda-lime glass. The metal mask covers the surface of the base glass except for the places where narrow channels are desired. The base glass covered with the mask is then immersed in a molten salt bath. The molten salt bath consists of a source of single valence ions, such as alkali metal, thallium or silver ions, which diffuse into the glass in the uncovered regions and replace the sodium ions at the glass surface. This results in a pattern of channels in the glass wherein the channels have higher density and altered electronic polarizability compared to surrounding regions. Both of these effects lead to a higher index of refraction, and thus to ion-exchanged channel waveguides in the glass substrate. In some cases, an electric field is applied to the glass sample during the ion-exchange process, in order to bury the waveguide below the surface of the sample.
Another technique used to form waveguides is photolithography. According to this technique, a suitable optical material is dissolved in a solvent, spin-coated onto a substrate, and exposed to ultraviolet light through a photomask. In a typical negative photolithographic process, the ultraviolet light causes the exposed portions of optical material to polymerize and harden. After this, the unpolymerized portions of the optical material are washed away to form the waveguide channels. In a typical positive photolithographic process, the ultraviolet light causes the exposed portions of the optical material to decompose while the unexposed portions harden. The decomposed portions are then washed away. In either case, a pattern of raised strips or channels of the optical material is left behind on the substrate.
Much interest has been expressed in waveguides formed from various polymers which offer non-linear and/or electrically controllable optical properties. Polymers also offer advantages of easy deposition and versatility. Nevertheless, applications of polymer waveguides have not yet achieved their full potential. This is believed to be largely due to the limitations of the existing production techniques which tend to produce rough surfaces which lead to unacceptably high energy losses, both along the length of the waveguides and particularly at input and output interfaces.
One approach to avoiding the need for photolithography when using polymer materials is proposed by U.S. Pat. No. 4,834,480 to Baker et al. This document proposes composite waveguides in which relatively shallow channels are defined in the surface of a glass substrate by ion-exchange and a uniform layer of polymer material is then deposited over the entire surface. The dimensions and properties are chosen such that light propagation can only occur via modes which overlap both the ion-exchange channels and the polymer layer, thereby ensuring that the transmission is affected by the properties of the polymer. This approach, however, is far from ideal for a number of reasons. First and foremost, the electromagnetic modes of this combined structure carry relatively small amounts of power inside the polymer. As a result, the desired non-linear properties of the waveguide are greatly reduced. In addition, the structure of the modes of this combined structure is rather unusual. This makes it difficult to achieve efficient coupling of the structure at an interface with a conventional single-mode optical fiber.
Finally, reference is made to an article entitled xe2x80x9cTunable Polymer Optical Add/Drop Filter for Multiwavelength Networksxe2x80x9d (C. Kostrzewa et al., IEEE Photonics Technology Letters, Vol. 9, No. 11, pp. 1487-1489, November 1997) which refers to a waveguide structure in which a silicon substrate with grooves formed by RIE was coated with polymer. This approach offers various advantages by employing RIE of the substrate to define the path of the waveguide. However, no solution is offered for achieving efficient coupling of such a waveguide to input or output fibers.
There is therefore a need for a waveguide production technique which would cheaply and simply produce polymer waveguides while reducing problems of rough waveguide surfaces. It would also be highly advantageous to provide a waveguide structure which would combine the advantageous properties of both polymer and ion-exchange-in-glass waveguides.
The present invention is a waveguide structure of polymer material on a non-polymer substrate and a corresponding production method.
According to the teachings of the present invention there is provided, a waveguide structure comprising: (a) a substrate made from non-polymer material which is substantially transparent with a refractive index n1 for electromagnetic radiation within at least one range of wavelengths, the substrate having an upper surface, at least one elongated channel being etched in the surface of the substrate, the elongated channel terminating at an end wall; and (b) a polymer material deposited within, and extending along substantially the entire length of, the at least one channel so as to be optically coupled to the end wall, the polymer material being substantially transparent with a refractive index n2 for electromagnetic radiation within the range of wavelengths, n2 being greater than n1 such that the polymer material defines a waveguide.
According to a further feature of the present invention, the elongated channel has at least one terminal portion adjacent to the end wall, the terminal portion defining a extension direction, wherein a region of the substrate extending from the end wall in the extensional direction is processed so as to exhibit a refractive index greater than n1, thereby defining a secondary waveguide portion optically coupled to the waveguide.
According to a further feature of the present invention, the substrate includes a lateral surface located so as to delimit the secondary waveguide portion, the lateral surface being polished to a roughness of no more than about 0.1 xcexcm.
According to a further feature of the present invention, the channel has a channel depth and wherein the polymer material is deposited within the channel as a layer having a thickness less than the channel depth.
According to a further feature of the present invention, the non-polymer material is selected from the group comprising: a glass material and a crystalline material.
According to a further feature of the present invention, the elongated channel has a substantially rectangular cross-section.
According to a further feature of the present invention, a cross-section of the elongated channel exhibits a concavely-rounded shape.
According to a further feature of the present invention, there is also provided a coating layer deposited over both the surface of the substrate and the polymer material, the coating layer having a refractive index smaller than n2.
There is also provided according to the teachings of the present invention, a method for producing a polymer waveguide on a non-polymer substrate, the method comprising: (a) etching at least one channel in a surface of the substrate, the channel terminating at an end wall within the substrate; and (b) depositing a polymer material within the at least one channel so as to form a waveguide.
According to a further feature of the present invention, the etching is performed to form a given channel depth, and wherein the depositing is performed so as to form a layer of the polymer material having a thickness less than the given channel depth.
According to a further feature of the present invention, the etching is performed via a mask defining the at least one channel.
According to a further feature of the present invention, the etching is performed using a wet etching process.
According to a further feature of the present invention, the etching is performed using a dry etching process.
According to a further feature of the present invention, the depositing is performed using a substantially uniform deposition process.
According to a further feature of the present invention, the depositing is performed using a spin-coating deposition process.
According to a further feature of the present invention, at least one region of the substrate is processed through an ion exchange process so as to form a secondary waveguide adjacent to the at least one channel.
According to a further feature of the present invention, a layer of coating material is deposited over both the surface of the substrate and the polymer material.